In recent years, influence of radio waves on human bodies has been paid attention to. There have been reports about the effect of elevation in temperature of a humane body exposed to intensive radio waves, the effect of stimulus to nerves, and the influence of intensity of radio waves on human beings.
The average value of the amount of energy of radio waves transmitted from a radio apparatus such as a portable telephone or the like during a call and absorbed for 6 minutes by a specific site (particularly a head) of a human body (hereinafter referred to as SAR (Specific Absorption Rate) has been regulated by law since June 2002. It is necessary to reduce SAR to be not higher than the regulated value.
In order to reduce SAR, it will go well if the power of radio waves emitted from the radio apparatus is depressed. However, there is a problem that the depression of the power leads to deterioration in speech quality.
As a method for reducing SAR without depressing the power emitted from a radio apparatus, there is a method in which a conductive flat plate having a predetermined shape is short-circuited to a board through a conductor as shown in FIG. 22. FIG. 22 is a view showing the configuration of an antenna mounted on a background-art portable radio apparatus.
As shown in FIG. 22, this portable radio apparatus is constituted by an antenna element 2202, a feeding portion 2203 for feeding power to the antenna, a circuit board 2204, a shield casing 2205 covering the circuit board 2204, a conductive flat plate 2206, and a short-circuit conductor 2207 for short-circuiting one end of the conductive flat plate 2206 to the shield casing 2205. The antenna element 2202 is provided outside a housing 2201 while the feeding portion 2203, the circuit board 2204, the shield casing 2205, the conductive flat plate 2206 and the short-circuit conductor 2207 are provided inside the housing 2201.
The other end of the conductive flat plate 2206 is electrically released from the shield casing 2205, and the length of the conductive flat plate 2206 is selected to be ¼ wavelength of the operating frequency. Consequently, the impedance between the conductive flat plate 2206 and the shield casing 2205 approaches substantially zero in the short-circuited end and infinite in the open end. Thus, a high-frequency current hardly flows from the vicinity of the feeding portion 2203 to the conductive flat plate 2206 or the shield casing 2205. As a result, the quantity of radio waves radiated from the conductive flat plate 2206 or the shied casing 2205 is reduced so that SAR is reduced.